Reproduction of jellyfish

In the previous article, I reported
on these jellyfish, especially on their polyps and asexual reproduction.
Now, I intend to write "another story" mentioned in it. That is, the sexual
reproduction of the jellyfish.

I had reared the medusae for about one month from mid October 1999.
They were all given by Miyakawa who is a veteran amateur on this species;
two females were grown up from polyps in his aquarium tank, and the two
wild males were collected by him in Nagoya city, central Japan.

However, as a matter of fact, female and male individuals of this species
are usually not found at the same time in the same habitat. Some people
think that this is due to the lack of complete investigations, but the
sexual reproduction in nature may not be common.

Nevertheless, the sexual reproduction is very important. It helps to
improve the genetic diversity of the population, and information about
their development is valuable for the understanding of these creatures.
I hope that this article makes you interested in medusae.

Eggs

Pictured left are the eggs of freshwater jellyfish, C. sowerbyi.
They were spawned in 20th Oct. and they are about 0.1mm in diameter. They
were sunk in the bottle in which I rear medusae, and I could pick them
up in a watchglass.

I separated females and males and I tried artificial insemination. Various
eggs were observed as seen in this picture. But these all seem unfertilized
because I was not able to observe any cleavage this time.

The three eggs marked with a '*' were good,
whereas the other eggs were bad and cannot develop, I think. I don't know
the reason why there were differences in egg viability.

Cleavage!

On the other hand, I couldn't find any sperms of the medusae, or make any
eggs fertile. Although Miyakawa successfully fertilized eggs artificially,
I abandoned it and started to rear all medusae in the same bottle. This
is not a good method to observe the start of cleavage. In fact, I couldn't
observe the fertilization.

But luckily enough, near the midnight of the 27th Oct. 1999, I found
two embryos that were in the two-cell stage of cleavage! Pictures were
taken continuously until morning with intervals of about five minutes.
I show some of them here.

23:54

00:46

01:53

02:38

03:28

16:08 (The left is unfertilized).

Two developing embryos can be seen in these pictures (except
the last). The indicated times are when the photo was taken. Animation
of the left and the right embryo
are also available (each about 200KB GIF, 100x100 pixels). In these animations,
disturbances in the arrangement of cells are seen especially when they
divide.

(23:54) The embryos are in the two-cell stage with its distinctive shape.
This shape is also reported in Hydra and Spirocodon. I can
see a membrane in which the embryo is covered. (Look at the right embryo,
and the top photo). This membrane confuses me, because I hear that there
is no fertilization membrane in Hydrozoa, to which freshwater jellyfish
belongs.

(00:46) This is in the four-cell stage. The four cells are arranged in
two ways. Tetrahedron on the left and flat on the right.

(01:53) Probably in the eight-cell stage. The arrangement of the cells
are not in good order.

(02:38) 16-cell stage maybe. I cannot count the number of cells.

(03:28) 32-cell stage? The shape of the left embryo is strange. But it
recovered as a ball afterward.

(16:18) The cleavage is already finished. The left ball is an unfertilized
egg and the right is the embryo. They are very similar. This embryo didn't
swim at this moment.

Although eggs or planulae were seen several times, this is the only cleavage
I observed.

Planulae

A creeping planula (left) and a frustule (right)(same magnification)

After the cleavage, the embryo began to swim. I observed a ball-shaped
embryo swimming slowly in small circles. This swimming soon ceased. The
embryos began to elongate and creep on the substrate. The term "planula"
is applied to this swimming stage and creeping stage until they settle
somewhere and become polyps.

Lefthand animation shows a creeping planula on the watchglass. The interval
is ten minutes and total length is 80. The speed of the movement is variable
according to the cases.

You may think that you saw this kind of animation somewhere else. Yes,
I showed the movement of a "frustule", asexual larva, in the article "Freshwater
Jellyfish". Frustules from polyps are very similar to planulae in their
morphology and movement. Righthand picture is not a planula but a frustule.

But they are different in some points, I gather. At any rate, frustules
are far larger than planulae as you see in the pictures above. Actual length
of the frustule in this picture is about 0.95mm.

Tiny polyps from planulae

A tiny polyp from a planula (left) and a mature polyp (right).

Some planulae became tiny polyps in several days. They are very small.
Enlarged
image of a tiny polyp is also available (brightfield: 20KB). The magnification
on the photographs of a creeping planula, a frustule, and these two polyps
are all the same.

The mouth of a tiny polyp is about 0.1mm in its diameter! What do they
feed on? Protozoa? I have recently successfully fed them with rotifers,
something like Rotaria. An animation
is also available (ca.25min., 18frames: 59KB GIF). I hope I can grow them
up to mature polyps with these kind of creatures "generated naturally"
in the container.

Acknowledgements:

Ryuta Miyakawa and other members of the "jfish-ML", Wim van Egmond and
Micscape Editor, Dave Walker encouraged and helped me very much again.
I express my gratitude to them.

References:

Here I mention a paper dealing with the sexual reproduction and development
of this animal. If you know of other information about it, please tell
me. And for general information on the web, please look at my
previous article "Freshwater Jellyfish" too.

Payne, F. 1926. Further studies on the life history of Craspedacusta
ryderi, a freshwater hydromedusan. Biological Bulletin of the Marine
Biological Laboratory (Woods Hole, Mass.) 50: 433-443.